18 | MOLTEN METAL w w w. m m c p u b l i c a t i o n s . c o . u k Beliefs about fused magnesium chloride fluxes questioned Chlorine gas has largely been replaced by magnesium chloride fluxes as a means of reducing alkalis and oxides from aluminium melts. Two widely held beliefs concerning the composition and effects of these fused fluxes have developed in recent years and this article explains why they should be questioned*. Over the last fifteen years fused magnesium chloride fluxes have largely replaced chlorine gas as a means of reducing alkalis and oxides from molten aluminium. The change, hastened by environmental considerations, began after publication of an article by Beland(1) in 1995. This described a refining flux based on the binary system magnesium chloride-potassium chloride, where a magnesium chloride intermediate was the rate controlling species active in the removal of alkalis. Refining fluxes were at first based on the binary magnesium chloride-potassium chloride system with two low melting point eutectics at about 55 and 36 mole magnesium chloride. Later a revised binary diagram was accepted as a more accurate description of the magnesium chloride system with a third eutectic occurring at 31 mole. Accordingly, commercial products became accepted based on the three eutectics with magnesium chloride contents ranging from the slightly hypo eutectic 25% by weight up to the hyper eutectic 75% by weight. Two widely held beliefs, have developed about fused magnesium chloride fluxes. MgCl percentage The first belief is that the performance in terms of efficiency of sodium removal is directly related to the percentage of magnesium chloride in the fused flux. It is well established that in fused magnesium chloride-potassium chloride fluxes magnesium chloride is the active component participating in the reaction: MgCl2 + 2 Na = 2 NaCl + Mg KCl on the other hand plays no part because as KCl is more stable than NaCl, having a higher free energy of formation and based on this the belief existed that the performance in terms of efficiency of sodium removal should be higher the higher the percentage of magnesium chloride in the product. In a number of separate series of full scale casthouse trials involving direct comparisons under the same conditions it was demonstrated that the performance of 60%, 40% and 25% MgCl2 flux compositions all produce equivalent results. Final confirmation came when David de Young (2) published the results of an investigation carried out at the Alcoa Technical Centre into the same subject and showed that varying the % MgCl2 between 10% and 90% in fused magnesium chloride-potassium chloride fluxes had no influence on the rate of sodium removal as shown in fig.1. In explanation of this Dietze(3) has proposed that the concentration of MgCl2 in the molten salt *Article based on a paper being presented by MQP at the 2011 Australian Asian Pacific Aluminium Casthouse Technology Conference, Melbourne. 1. Effect of % MgCl2 on sodium removal efficiency (after De Young) droplet has little influence on the reaction kinetics because the rate of salt addition applied is ten times that needed to satisfy the requirement for stoichometric reaction. Of far greater importance are the kinetic factors and therefore in order to achieve an adequate distribution of a relatively small amount of flux in the aluminium melt it is necessary to add considerably more than the required stoichometric amount. Thus the concentration of MgCl2 in the individual salt droplets, providing that it is always greater than that required to satisfy the reaction, has little influence on the reaction kinetic. The conclusion from such comprehensive trials and examination of reaction kinetics is that the performance in terms of efficiency of sodium removal is not related to the percentage of magnesium chloride in the fused flux. 2. Ternary system MgCl2 – KCl – NaCl showing new salt flux composition Aluminium Times September 2011 Sodium chloride content The second widely held belief is that the amount of sodium chloride permitted in fused salt products for sodium removal from aluminium melts should be restricted below 1 per cent. When fused salt fluxes were first introduced potassium chloride was a suitable low cost material to combine with the more expensive MgCl2 component to provide a low melting point eutectic. However, this dramatically changed with the price of KCl increasing by more than 350% up to a peak of nearly US$ 900 per tonne in July 2009 due to global demand for potash fertiliser for the production of grain crops and biofuels. It seems inevitable that KCl prices will rise again resulting in cost increases for both producers and users of fused salts. This has led to a reassessment on the possible role of NaCl in fused salts 20 | MOLTEN METAL where it would appear to be an ideal substitute or partial substitute for KCl. The reaction, MgCl2 + 2 Na = 2NaCl + Mg, can be considered to move strongly to the right which means that sodium removal by MgCl2 is practically effective. Ellingham diagrams confirm the high stability of NaCl with respect to MgCl2. It can be anticipated that in practice there would be no increase in sodium in the aluminium if quantities of NaCl were introduced into the flux composition. The above hypothesis was tested by thermodynamic modeling of the reaction. A ternary product composition was selected, with an appropriate addition of NaCl to an existing proven formulation, by reference to the ternary diagram in fig 2. A thermodynamic modelling study was undertaken with IME Aachen to investigate the effect of a sodium chloride content from 1 to 25 % in a magnesium–potassium chloride fused flux, on the removal of up to 30 ppm of sodium from an aluminium melt. Modelling showed that no sodium remained in the melt after reaction and therefore it can be concluded on the basis of the thermodynamic calculations that up to 25% NaCl can be substituted for KCl in Refinal without any effect on the residual Na content after treatment. Full scale casthouse trials were then carried out at a major casthouse in Europe by comparing Refinal, containing 25% sodium chloride, with the standard practice using a conventional Refinal 35% MgCl2, 65% KCl fused flux. Over a total of 87 furnace preparations an average Na level of 6.5 ppm at first time of batching was achieved with the NaCl containing flux which was equivalent to the results achieved with their standard practice. The conclusion from thermodynamic modelling and casthouse trials is that sodium chloride can be added to fused magnesium chloride-potassium chloride fluxes without affecting sodium removal from aluminium melts. Sensible beliefs It is not surprising that it was believed that the performance of a flux might be proportional to MgCl2 content. Nor is it surprising that it was believed that adding NaCl to a flux might lead to Na pick up, when the objective is to remove Na. Nonetheless impartial scientific investigation has confirmed otherwise in both cases. In the case of the effect of MgCl2 the reaction kinetics are of overriding importance and not the proportion of MgCl2 in the product. In the case of the effect of NaCl addition, the thermodynamics confirm that irrespective of kinetic considerations NaCl cannot be reduced again to Na in the reaction system involved. Bibliography G.Beland, C. Dupuis and J.-P. Martin, “Improving fluxing of Aluminium Alloys” Light Metals, 1995, pp1189-1195. (2) D.H.DeYoung, “Salt Fluxes for Alkali and Alkaline Earth removal from Molten Aluminium”, 7th Australian Asian Pacific Conference Aluminium Casthouse Technology, 2003. (3) Private communication with Dr. A. Dietze & TU Clausthal (1) Reader Reply No.48 Author: John Courtenay and Michael Bryant, both of MQP Ltd, UK w w w. m m c p u b l i c a t i o n s . c o . u k Thermserve billet heater for Vimetco in Slatina Thermserve Limited, based at Telford in the United Kingdom have recently moved to larger premises which allows the company to not only design equipment and machinery but to also engineer and build. The company recently was awarded the Queen’s Award for Enterprise Vimetco Extrusion SRL, with their headquarters in Slatina, in Romania, is a subsidiary of VimetcoALRO S.A. which is one of the largest smelters in Europe. The smelter has been operating since 2006 and Vimetco came to Thermserve for a 2500Kg/hr 178mm billet heating furnace and hot log saw with specific performance requirements concerning throughput and fuel consumption. The equipment is currently undergoing Factory Acceptance Tests in the Thermserve fabrication shop, in the United Kingdom. Thermserve log and billet ovens are manufactured in their own, well equipped, fabrication facility. For ease of construction, maintenance and to accommodate potential extensions to the service requirements Thermserve use modular design techniques. For this reason, the sectional mild steel casing will have readily removable top sections to access the pre cast refractory blocks and roller conveyor system which transport the logs through the furnace. These steel and precast refractory sections are constructed with all the necessary apertures for the various inserted services and instrumentation. The insulation materials including the precast refractory blocks are designed to give thermal efficiency through the full range of the oven capabilities. The combustion equipment consists of a range of burners and is divided into independently controlled zones, which are coordinated with the overall modular design of the oven. Each zone is equipped with all the required safety systems, including pilot burners and flame failure devices. A recuperative section provides recycling of the hot gasses ensuring efficient combustion and heat transfer. Close to the discharge end of the recuperative zone and within each fired zone Thermserve provide pneumatically operated thermocouples to contact the surface of the log. The operation of the thermocouples is synchronised with the passage of the log through the furnace and sequenced by the PLC control system as with the main body of the oven the combustion system is design with ease of inspection and routine maintenance in mind. Essential to the efficient operation of the log/billet heater is the functional furnace pressure control to maintain a balanced pressure within the heating chamber and eliminate cold air entry, therefore an electronic transmitter is connected to a sensing point near the discharge end. The transmitter actuates an exhaust damper through an electrical modulating motor, so that the damper closes when the burners are turned down, and opens as the burners move to maximum firing rate. Pressure control is assisted by the provision of an actuated, insulated door at the discharge end of the furnace where a photocell is located to control movement for the pusher gear and to stop each log in a fixed Aluminium Times September 2011 Combustion system controls. position. Thus no adjustment of controls is necessary when changing log length. All of the combustion and control features of a Thermserve log/billet oven are designed to provide the required level of information to the control and Instrumentation centre in order to provide intelligent temperature control. The control centre is designed, built and thoroughly tested in Thermserve’s in house panel shop to optimise the temperature and to manage the in feed system with the following features:• Automatic temperature control for each of the fired zones of the furnace. • Control of all log movement sequencing from storage through the heater. • Integration of furnace and handling equipment. • An audible alarm that will sound in the event of abnormal temperature condition in any zone. • Full integration of a Thermserve Hot Saw In this case, Allen Bradley PLC equipment, is specified but similar equipment by other manufacturers can be utilised if requested. The Thermserve main panel also houses the following:• Isolator, giving mechanical and electrical interlocks with the cubicle door. • Direct on-line motor starters, overloads, fuses, etc. • Flame failure relay and ignition units for the fired zones. * HMI – Push buttons, labels, indicator lights, etc., for control supply and emergency stops. • The panel is fully wired to terminal strips and shop tested before despatch. The equipment for this project has been designed and manufactured to relevant British Standards (or European Standards EN/ISO - as appropriate) To facilitate an efficient installation the log heater is currently fully assembled in the Thermserve works and will be dismantled only to the extent necessary for efficient and safe transportation to site. Reader Reply No.49
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